Elsevier

Journal of Luminescence

Volume 229, January 2021, 117668
Journal of Luminescence

Photophysical properties of novel fluorescent thin solid layers based on the Aggregation Induced Emission of alkoxy-substituted salicylaldehyde azines

https://doi.org/10.1016/j.jlumin.2020.117668Get rights and content

Highlights

  • An uniform, fluorescent thin films using solvent methods have been created.

  • Self-organization of the molecules in the layer depends on the alkoxy substituent.

  • Small-molecule SAAs derivatives can be successfully used for the preparation of ink.

  • The production of a fluorescent layer with a modern printing method has been shown.

Abstract

A series of salicylaldehyde azines substituted with linear and branched alkoxy chains were synthesized and characterized. All derivatives exhibited photoluminescence in the thin layers due to Aggregation Induced Emission (AIE) properties. Ability to self-organizing of the azines in the thin solid layer deposited via evaporation, spin-coating and inkjet printing techniques, due to the nature of the substituent was investigated. The electronic properties, that is, orbital energies and resulting energy gap were calculated theoretically by density functional theory (DFT). On the selected examples inkjet inks were prepared and printed smooth uniform fluorescent layers appropriate for organic light emitting diodes (OLEDs).

Introduction

Luminogenic materials with unique Aggregation Induced Emission (AIE) have attracted growing attention as materials with broad spectrum of application in various technological fields [1]. Conventional luminescent materials used in the field of organic electronics, especially in organic light emitting diodes (OLEDs), transistors, organic lasers, as well as fluorescent sensors or probes have suffered from aggregation caused quenching (ACQ) due to intermolecular interactions when their molecules are in the close proximity [1,2]. In the field of OLEDs, aggregation is inhibited e.g. by so-called host-guest systems, when small-molecule emitters are dispersed in a polymer matrix [[3], [4], [5]]. It forces us to use emitters with very high quantum yields and with specified HOMO/LUMO levels to interact with the matrix [5].

In sharp contrast to destructive ACQ, Aggregation-Induced Emission (AIE) enables active utilization of the aggregation process, instead of passively working against it. AIE refers to the group of luminogenic materials which emission is poor once they are dissolved in appropriate solvents but become significantly increased when aggregates are formed in solid state or in the thin layer. The most possible working mechanism involved in this unique and useful process is called restriction of intramolecular rotation (RIR) which could be achieved through bulky substitutions, metal chelation or strong hydrogen bonding [1,2].

Salicylaldehyde azines (SAAs) are typical molecules with AIE process. SAAs are highly stable condensation products with hydrazine which display several advantages such as easy synthesis and product purifications, low costs or high melting points [[6], [7], [8]]. The active hydrogen of phenolic hydroxyl group in salicylaldehyde derivatives plays an important role of AIE process, and determines the fluorescence ability in aggregation state. Additionally, SAAs exhibit Excited State Intramolecular Proton Transfer (ESIPT) involving a photo-induced enol-keto tautomerization [9]. The basic photophysical properties of the AIE azines with ESIPT process have been shown in the literature [[10], [11], [12] ]] and are illustrated in Fig. 1. Azines exist in the enol form at the ground state, in which the intramolecular hydrogen bond is formed. Upon photoexcitation, the singlet excited state of the enol form is produced. Then, an ultrafast ESIPT process occurs and the keto form at the singlet excited state is formed, which again is stabilized by the intramolecular hydrogen bond. The fluorescence observed in azine systems comes from keto tautomers. It was known that AIE-ESIPT emission was highly solvent dependent due to its influence on hydrogen bonding, whereas in solid state we only observed emission from the keto form [11]. AIE-ESIPT salicylaldehyde azines have drawn much attention due to the wide application possibilities such as optoelectronics [13], chemosensors [[14], [15], [16]] or for bio-imaging [[17], [18], [19]].

Luminescent materials tend to be crystalline and have low solubility in organic solvents. The presence of alkyl chains attached to the main core play an important role in molecular self-assembly by regulating molecular stacking and solubility in organic solvents. This properties are essential for investigations of photophysical and assembly properties [20,21]. Changing the length of alkyl chains is the most facile strategy for assembly structure regulation, e.g. short chains have small steric hindrance causing π – π stacking interactions disfavouring AIE. Whereas long chains promote free intramolecular motions, the redundant flexibility hinders regular agglomeration [22]. The use of, apart from linear chains, also the branched ones can significantly affect the properties of thin layers of a solid. Branched alkyl chains typically have quite different properties (e. g. melting point, viscosity, density) from their linear counterparts [23].

The main methods of producing thin layers of solids include the spin-coating method and vacuum evaporation [24,25]. Spin-coating is solution processes which means that the compound is applied from a solution and is one of the chemical methods of obtaining layers. During spin-coating, evaporation, causing a reduction in the thickness of the layer, hence a very important element is the selection of appropriate process conditions. Spin-coating requires dissolving the compound in an organic solvent, therefore this method has another limitation regarding the solubility of the substance. In vacuum evaporation technique the particles are applied as a solid from the gaseous phase. The development of a solvent-free method of applying thin films is a very important prerequisite for the large-scale use of such materials, since the evaporation of organic solvents is environmentally unfriendly and entails high costs [26]. In addition, vacuum evaporation also has other advantages, such as the ability to accurately control the process by controlling the temperature and the amount of carrier gas. The disadvantages of this method, however, include the high cost of specialized equipment. An alternative to both methods can be printing method (ink-jet printing) which will simplify the technological process and significantly reduce its costs. In comparison with vacuum deposition, solution-processed film preparation techniques, including ink-jet printing is more feasible to manufacture large-area OLEDs with less material [27]. This method has been used in photovoltaics and organic electronics due to the fact that it is a non-contact printing method - it is suitable for large-format printing on a selected substrate as well as for precision printing [28]. It should be emphasized that the substrate can be of any shape and can be flexible. At present, ink-jet printing is the most dynamically developing printing technique, which allows to control the layers formed and ensures the appropriate speed of coating at a reasonable cost.

In the present paper, the synthesis and use of alkoxy-substituted salicylaldehyde azines (SAAs) in the fabrication of AIE fluorescent thin layers are described. We have designed the target molecules to connect highly fluorescent core with long and/or branched alkyl chains. Modifications around the core of the designed systems make them applicable to the production of fluorescent thin layers, which can be used as luminescent materials as well as organic semiconductors, e.g. in OLEDs. It is particularly important to obtain derivatives that will be solution processable to implement the inkjet printing techniques for the fabrication of the thin layers [[29], [30], [31], [32]]. For the received systems, HOMO/LUMO levels and energy gaps (Eg) were calculated theoretically using density functional theory (DFT), the photophysical properties as well as the ability to thin film formation were tested [[33], [34], [35], [36], [37], [38]].

Section snippets

Materials

All chemicals and solvents for the synthesis of salicylaldehyde derivatives were purchased from Merck Company and used without additional purification.

Synthesis and analysis

Symmetrical alkoxy-substituted azines 1 were obtained from appropriate salicylaldehyde derivative 2 and hydrazine hydrate according to general method described in the literature (Scheme 1) [6,8,39]. As a starting material in the synthesis of alkoxy-substituted salicylaldehyde derivatives 2 2,4-dihydroxybenzaldehyde 3 was used. After optimization

DFT calculations

For proper design of OLED devices proper LUMO/HOMO energy levels of active layers are important parameters. Theoretical calculations had primarily three objectives: optimization of ground state geometry, emergence of compounds with a large HOMO/LUMO gap and determination of the possibility of charge transfer by a given molecule. The calculations were carried out with Gaussian 16 [33] and SCIGRESS 3.2.2 programs [34]. Quantum theoretical calculations using density functional theory (DFT) were

Conclusions

The straightforward synthesis of a series of substituted salicylaldehyde azines and their application as efficient luminophores showing a photoluminescence quantum yields approaching from 0.8% in solution to 23% in thin film were described. The introduction of alkoxy chains into the fluorescent core caused slight changes in the fluorescence of the obtained systems which exhibited a tendency to self-assemble, forming high quality thin layers and depending on the alkoxy substituent. It turned out

Author statement

Justyna Anna Adamczyk, Investigation; Writing - original draft; Karolina Zielonka, Investigation; Writing - original draft; Sylwia Kotarba, Investigation; Methodology; Writing - original draft; Writing - review & editing, Jakub Saramak, Investigation; Methodology; Writing - original draft; Ireneusz Glowacki, Methodology; Writing - review & editing, Michał Rachwalski, Supervision; Writing - review & editing, Adam Marek Pieczonka, Conceptualization; Investigation; Methodology; Supervision;

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

Financial support by the Ministry of Science and Higher Education (MNiSW) in Poland, Grant Iuventus Plus nr IP2014–035873 for A.M.P., is gratefully acknowledged.

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